1. Field of the Invention
The present invention relates to a display device, a driving method of a display device, and an electronic device, and particularly to a flat type (flat panel type) display device formed by arranging pixels including an electrooptic element in the form of a matrix, a driving method of the display device, and an electronic device having the display device.
2. Description of the Related Art
In a field of display devices for making image display, flat type display devices formed by arranging pixels (pixel circuits) including a light emitting element in the form of a matrix have recently been spreading rapidly. The development and commercialization of a flat type display device using a so-called current-driven type electrooptic element changing in light emission luminance according to the value of current flowing through the device as the light emitting element of a pixel, for example an organic EL display device using an organic EL (Electro Luminescence) element utilizing a phenomenon of light being emitted when an electric field is applied to an organic thin film as the light emitting element of a pixel have been under way.
The organic EL display device has the following features. The organic EL element can be driven by an application voltage of 10 V or lower, and thus consumes low power. In addition, because the organic EL element is a self-luminous element, the organic EL display device provides high image visibility as compared with a liquid crystal display device that displays an image by controlling the intensity of light from a light source (backlight) in a liquid crystal cell in each pixel including the liquid crystal cell. Moreover, because an illuminating member such as the backlight or the like essential to the liquid crystal display device is not demanded, the organic EL display device can be easily reduced in weight and thickness. Further, because the organic EL element has a very high response speed of a few μsec or so, no afterimage occurs at a time of displaying a moving image.
As with the liquid crystal display device, the organic EL display device can adopt a simple (passive) matrix system and an active matrix system as driving system of the organic EL display device. However, while having a simple structure, a simple matrix type display device presents, for example, a problem of difficulty in realizing a large and high-definition display device because the emission period of an electrooptic element is reduced by an increase in the number of scanning lines (that is, the number of pixels).
Therefore an active matrix type display device that controls current flowing through an electrooptic element by an active element, for example, an insulated gate field effect transistor (typically a TFT (Thin Film Transistor)) provided within a same pixel circuit as the electrooptic element, has recently been actively developed. The active matrix type display device makes it easy to realize a large and high-definition display device because the electrooptic element continues emitting light over the period of one frame.
It is generally known that the I-V characteristic (current-voltage characteristic) of the organic EL element is degraded with the passage of time (so-called secular degradation). In a pixel circuit using an N-channel type TFT as a transistor that drives an organic EL element (which transistor will hereinafter be described as a “drive transistor”), the organic EL element is connected to the source side of the drive transistor. Thus, when the I-V characteristic of the organic EL element is degraded with the passage of time, the gate-to-source voltage Vgs of the drive transistor changes. As a result, the light emission luminance of the organic EL element also changes.
This will be described more specifically. The source potential of the drive transistor is determined by an operating point of the drive transistor and the organic EL element. When the I-V characteristic of the organic EL element is degraded, the operating point of the drive transistor and the organic EL element varies. Thus, even when a same voltage is applied to the gate of the drive transistor, the source potential of the drive transistor changes. Thereby, the gate-to-source voltage Vgs of the drive transistor changes, and therefore the value of current flowing through the drive transistor changes. As a result, the value of current flowing through the organic EL element also changes, so that the light emission luminance of the organic EL element changes.
Further, in a pixel circuit using a polysilicon TFT, in addition to a secular degradation in the I-V characteristic of an organic EL element, there occur secular changes in threshold voltage Vth of a drive transistor and in mobility μ of a semiconductor thin film forming the channel of the drive transistor (which mobility will hereinafter be described as “mobility of the drive transistor”), and there is a difference in the threshold voltage Vth and mobility μ in each pixel due to variations in a manufacturing process (there are variations between the characteristics of individual transistors).
When the threshold voltage Vth and the mobility μ of the drive transistor differ in each pixel, the value of current flowing through the drive transistor varies in each pixel. Thus, even when a same voltage is applied to the gates of respective drive transistors in different pixels, light emission luminance varies between respective organic EL elements in the different pixels. As a result, screen uniformity is impaired.
Accordingly, in order to hold the light emission luminance of the organic EL element constant even when a secular degradation occurs in the I-V characteristic of the organic EL element or a secular change occurs in the threshold voltage Vth or the mobility μ of the drive transistor, without being affected by the secular degradation in the I-V characteristic of the organic EL element or the secular change in the threshold voltage Vth or the mobility μ of the drive transistor, a constitution is adopted, which constitution provides each of pixel circuits with a function of compensating for variations in the characteristic of the organic EL element and correcting functions of correcting for variations in the threshold voltage Vth of the drive transistor (which correction will hereinafter be described as “threshold value correction”) and correcting for variations in the mobility μ of the drive transistor (which correction will hereinafter be described as “mobility correction”) (see Japanese Patent Laid-Open No. 2006-133542, for example).
By thus providing each pixel circuit with the function of compensating for variations in the characteristic of the organic EL element and the correcting functions of correcting for variations in the threshold voltage Vth and the mobility μ of the drive transistor, the light emission luminance of the organic EL element can be held constant even when a secular degradation occurs in the I-V characteristic of the organic EL elementor a secular change occurs in the threshold voltage Vth or the mobility μ of the drive transistor, without being affected by the secular degradation in the I-V characteristic of the organic EL element or the secular change in the threshold voltage Vth or the mobility μ of the drive transistor.